Field of the Invention
The present invention relates generally to a polymer electrolyte battery provided with a positive electrode, a negative electrode, and a polymer electrolyte, and a method of fabricating the polymer electrolyte battery, and more particularly, to a polymer electrolyte battery characterized in that a polymer electrolyte is modified to improve charge/discharge cycle performance of the battery.
Description of the Related Art
Recently, as one type of advanced batteries featuring high power and high energy density, non-aqueous electrolyte batteries with high energy density have been used. The non-aqueous electrolyte battery employs a non-aqueous electrolyte solution and utilizes a process of oxidation and reduction of lithium and lithium ions.
In the case of the above-mentioned non-aqueous electrolyte battery, however, problems exist that the non-aqueous electrolyte solution leaks out of the battery and that the non-aqueous electrolyte solution reacts with a positive electrode or a negative electrode to degrade the battery characteristics.
Therefore, more recently, the spotlight is on a polymer electrolyte battery employing a polymer electrolyte comprising a polymer-based material containing an electrolyte or a non-aqueous electrolyte solution.
Such a polymer electrolyte battery has conventionally generally employed a polymer electrolyte comprising a polymer-based material such as poly(ethylene oxide) and polyvinylidene fluoride containing a lithium salt such as LiPF6 as an electrolyte, or a gelated polymer electrolyte obtained by impregnating the above-mentioned polymer-based material with a non-aqueous electrolyte obtained by dissolving the above-mentioned electrolyte in an organic solvent such as carbonic ester.
However, poly(ethylene oxide), polyvinylidene fluoride, or the like, which has been conventionally utilized as a polymer-based material, generally suffers low ion conductivity and poor chemical stability and hence, charge/discharge cycle performance of the polymer electrolyte battery is degraded.
Therefore, in recent years, in order to improve the ion conductivity and the chemical stability of a polymer-based material, a polymer electrolyte battery employing the polymer-based material obtained by coporimerizing two types of (meth)acrylate or acrylate has been proposed, as disclosed in Japanese Patent Laid-Open No. 32022/1995.
Unfortunately, however, even when a copolymer disclosed in the gazette is used as a polymer-based material, the ion conductivity and the chemical stability of the polymer-based material are not sufficiently improved. Further, when the polymer-based material is gelated by being impregnated with a non-aqueous electrolyte solution, the polymer-based material can not sufficiently hold the non-aqueous electrolytic solution, whereby the charge/discharge cycle performance of the polymer electrolyte battery cannot be sufficiently improved.
A first object of the present invention is to improve, in a polymer electrolyte battery provided with a positive electrode, a negative electrode, and a polymer electrolyte, the ion conductivity and the chemical stability of the polymer electrolyte.
A second object of the present invention is to enable the above-mentioned polymer electrolyte to sufficiently hold a non-aqueous electrolyte solution.
A third object of the present invention is to improve charge/discharge cycle performance of a polymer electrolyte battery.
In a polymer electrolyte battery provided with a positive electrode, a negative electrode, and a polymer electrolyte, a polymer electrolyte battery according to the invention is characterized in that a polymer-based material containing a copolymer of ethylene glycol (meth)acrylate compound represented by the following general formula (1) and alkyl (meth)acrylate represented by the following general formula (2) is used as said polymer electrolyte. 
As in the polymer electrolyte battery according to the present invention, when the polymer-based material containing a copolymer of the ethylene glycol (meth)acrylate compound represented by the foregoing general formula (1) and the alkyl (meth)acrylate represented by the foregoing general formula (2) is used as the polymer electrolyte, the ion conductivity and the chemical stability of the polymer electrolyte are improved and a non-aqueous electrolyte solution is sufficiently held in the polymer electrolyte. As a result, resistance on the surfaces at which the polymer electrolyte contacts with the positive electrode and the negative electrode is reduced so that charge/discharge cycle performance of the polymer electrolyte battery is improved.
Further, when ethylene glycol (meth)acrylate compound represented by the foregoing general formula (1) wherein R1 indicates an alkyl group having 9 or more carbon atoms is used as said ethylene glycol (meth)acrylate compound represented by the general formula (1), it is considered that alkyl groups are tangled with each other to form a three-dimensional bridge structure, so that the stability and the uniformity of the polymer electrolyte is improved, whereby the charge/discharge cycle performance of the polymer electrolyte battery is further improved.
When an ethylene glycol chain in said ethylene glycol (meth)acrylate compound is too long, a copolymer containing the ethylene glycol (meth)acrylate compound is dissolved in a non-aqueous electrolyte solution. Therefore, it is preferable to use ethylene glycol (meth)acrylate compound represented by the foregoing general formula (1) wherein n indicates an integer of 1 to 25.
On the other hand, when alkyl (meth)acrylate represented by the foregoing general formula (2) wherein R2 indicates an alkyl group having not more than 3 carbon atoms is used as said alkyl (meth)acrylate, the polymer-based material obtained is dissolved in the non-aqueous electrolyte solution. Therefore, it is necessary to use alkyl (meth)acrylate represented by the foregoing general formula (2) wherein R2 indicates an alkyl group having 4 or more carbon atoms. In order to further prevent the polymer-based material from being dissolved in the non-aqueous electrolyte solution, it is preferable to use alkyl (meth)acrylate represented by the foregoing general formula (2) wherein R2 indicates an alkyl group having 6 or more carbon atoms.
Further, in the polymer electrolyte battery according to the present invention, as the above-mentioned polymer electrolyte, it is possible to use a solid polymer electrolyte comprising a polymer-based material containing an electrolyte, or a gelled polymer electrolyte comprising a polymer-based material impregnated with a non-aqueous electrolyte solution obtained by dissolving an electrolyte in an organic solvent.
As an electrolyte used in a polymer electrolyte, it is possible to use a known electrolyte which has been conventionally used. Examples of such an electrolyte include lithium compounds such as lithium hexafluorophosphate LiPF6, lithium perchlorate LiClO4, lithium tetrafluoroborate LiBF4, and lithium trifluoromethanesulfonate LiCF3SO3. Specifically, in order to prevent the polymer electrolyte from being decomposed, it is preferable to use an imido electrolyte represented by LiN(CmF2m+1SO2)2. When the molecular weight of the imido electrolyte is too large, the ion conductivity of the polymer electrolyte battery is degraded. Therefore, it is preferable to use the imido electrolyte represented by the above-mentioned formula wherein m indicates an integer of 1 to 4. For example, lithium trifluoromethanesulfonimide LiN(CF3SO2)2 is preferably used.
As a solvent dissolving the above-mentioned electrolyte, it is possible to use known solvents which have been conventionally used. For example, an organic solvents such as propylene carbonate, ethylene carbonate, y-butyrolactone, butylene carbonate, 1,2-dimethoxyethane, dimethyl carbonate, diethyl carbonate, and the like can be used. These solvents may be used alone or in combination of two or more types.
In the polymer electrolyte battery according to the present invention, it is possible to use, as a positive electrode material for use in its positive electrode, a known positive electrode material that has been conventionally generally used. Examples of a usable positive electrode material include transition metal compounds capable of occluding and discharging lithium ions, which are represented by metal oxides containing at least one of manganese, cobalt, nickel, vanadium, and niobium and the like. Specifically, LiCoO2, LiNiO2, LiMnO2, and the like can be used.
As a negative electrode material for use in its negative electrode, it is possible to use a known negative electrode material that has been conventionally generally used. Examples of a usable negative electrode material include carbon materials capable of occluding and discharging lithium ions such as artificial graphite and natural graphite, lithium metals, lithium alloys, Li4Ti5O12, and TSi2.
Further, in fabricating the polymer electrolyte battery according to the present invention, the battery can be fabricated in such a manner that ethylene glycol (meth)acrylate compound represented by the foregoing general formula (1) and alkyl (meth)acrylate represented by the foregoing general formula (2) are copolymerized to fabricate a polymer electrolyte, after which the polymer electrolyte is contained in the battery so as to be sandwiched between a positive electrode and a negative electrode, or in such a manner that ethylene glycol (meth)acrylate compound represented by the foregoing general formula (1) and alkyl (meth)acrylate represented by the foregoing general formula (2) are copolymerized to fabricate a polymer electrolyte in the battery. When the polymer electrolyte is fabricated in the battery as described above, resistance on the surfaces at which the polymer electrolyte contacts with the positive electrode and the negative electrode is further reduced so that the battery characteristics are improved.